The focus of this application is to determine the mechanisms by which adenovirus (Ad) transforming E1A protein induces progression to S phase in quiescent cells, in a p300 binding dependent manner. In cooperation with cellular activated ras or virus encoded E1B19K or 55K proteins and by binding to p300/CBP and Rb family proteins, E1A can transform rodent cells. Interactions of E1A with p300/CBP and Rb proteins in quiescent cells result in rapid induction of S phase. While the inactivation of Rb proteins is well known to induce E2F and S phase, the contribution of E1A binding to p300 has not been thoroughly studied. Previously we showed that p300 prevents premature entry of quiescent cells into S phase by negatively regulating c-Myc. p300 cooperates with transcription factor YY1 and histone deacetylase 3 at an upstream YY1 binding site to keep c-Myc in a repressed state and that this contributes to maintaining cells in G0 phase. E1A induces c-Myc by interfering with this mechanism. Recently, we have discovered that a number of additional genes related to DNA synthesis and cell cycle progression are also repressed by p300 in quiescent cells and these genes are induced by E1A in a p300 binding dependent manner. Either downregulation of p300 by shRNAs or expression of wild type E1A but not p300 binding defective E1A mutants leads to the induction of c-Myc that is linked to rapid induction of S phase, activation of DNA damage response and S phase arrest. These cells contain increased cellular DNA replication origin activity. We now propose studies to determine how the origin activity is deregulated by E1A in a p300 binding dependent manner. These studies are expected to shed new light on E1A mediated aberrant S phase progression and its role in viral and cellular transformation. PUBLIC HEALTH RELEVANCE: Deregulation of cell growth control by a variety of mechanisms is a key step in the development of human cancer. Adenovirus encoded E1A is a time honored model oncoprotein. Understanding its effects on aberrant S phase progression will advance our understanding of how an oncogene by disrupting the growth inhibitory pathways of a cellular protein contributes to neoplastic transformation.